1. Field of the Invention
The present invention relates to a method for charging a nonaqueous electrolyte secondary battery, which is capable of keeping a nonaqueous electrolyte secondary battery in a charged state for a long period of time, and a charging apparatus for carrying out the charging method.
2. Related Art
Secondary batteries are rechargeable and can be repeatedly used, and therefore can contribute to waste reduction. Such secondary batteries are widely used as power sources of portable electric devices independent of AC power and as backup power sources in case of AC power failure. In recent years, secondary batteries have come to be used in a wider range of applications, and therefore there has been a growing demand for higher-performance secondary batteries having large capacity, excellent temperature characteristics, and high safety.
As such secondary batteries, lead-acid batteries, nickel-cadmium secondary batteries, nickel-hydride secondary batteries, and nonaqueous electrolyte secondary batteries, and the like have been developed and are used throughout the world. Among these secondary batteries, nonaqueous electrolyte secondary batteries are compact and lightweight and have large capacity, and are therefore widely used in not only compact personal computers and mobile phones but also digital cameras and video cameras.
Such a nonaqueous electrolyte secondary battery currently used has a cathode, an anode, and an electrolyte. The cathode can be composed of, for example, a lithium-containing cobalt composite oxide or a lithium-containing nickel composite oxide. The anode can be composed of an anode active material such as a carbon-based material (e.g., a graphite-based material or a coke-based material). The electrolyte can be obtained by dissolving a lithium salt such as LiPF6 or LiBF4 in an organic solvent such as a cyclic carbonate or a chain carbonate. The cathode and anode are formed into a sheet shape, and are impregnated with the electrolyte. Further, between the cathode and the anode, a separator is interposed to electronically insulate the cathode and the anode from each other. These cathode, anode, and separator are housed in a container having any shape.
Nickel-cadmium secondary batteries and nickel-hydride secondary batteries have a reaction path for allowing a charging current externally applied thereto after the end of charging to escape as heat from the system. For this reason, these secondary batteries have overcharge tolerance, and are therefore generally trickle-charged by continuously applying a minute current to them after full charge to keep them full charged.
On the other hand, nonaqueous electrolyte secondary batteries do not have a reaction mechanism for allowing a current causing overcharge to escape as heat from the system, and are therefore more likely to be overcharged and significantly degraded in battery performance as compared to nickel-cadmium secondary batteries and nickel-hydride secondary batteries. In addition, if a large current is continued to be applied to an overcharged nonaqueous electrolyte secondary battery, there is a possibility that safety problems will arise. In order to solve the above problems, various charging methods alternative to trickle charging, such as intermittent charging and constant-voltage charging have been proposed, but they still have problems in that the required amount of charge cannot be continuously ensured and significant degradation of batteries occurs. For example, JP-A 2000-236632 (KOKAI) discloses a method for minimizing the degradation of a nickel-hydride battery by controlling the time interval between charges and a charging current during intermittent charging of the battery.
However, when a nonaqueous electrolyte secondary battery is charged with a constant current easily obtained during main charging (i.e., during charging carried out with a first level of charging current and charging carried out with a second level of charging current in the charging method disclosed in JP-A 2000-236632 (KOKAI)), there is a possibility that a battery voltage becomes too high, thereby degrading the battery and impairing heat stability of the battery. Further, when a nonaqueous electrolyte secondary battery is charged in a rest period by constant-voltage charging disclosed in JP-A 2000-236632 (KOKAI), the battery is kept at a predetermined high voltage for a long period of time, thereby causing a problem that battery characteristics are significantly deteriorated.
In addition, a nonaqueous electrolyte secondary battery generally uses an electrolyte mainly containing an organic solvent, and therefore as in the case of a nickel-hydride battery using an aqueous solution, there is a danger that the battery will expand or explode as a result of an increase in the vapor pressure of the organic solvent or vaporization of the organic solvent caused by temperature rise. Further, the organic solvent such as a carbonate is flammable, and therefore if a liquid or gas containing the organic solvent leaks from the battery, there is a danger of ignition or explosion.
In order to solve the above problems, use of an ionic liquid having no flash point as an electrolyte has been studied to significantly improve safety of nonaqueous electrolyte secondary batteries. By using such an ionic liquid as an electrolyte, it is possible to obtain a battery which can operate in a high temperature environment where even a battery using an aqueous solution as an electrolyte cannot operate as well as to overcome the problem of conventional nonaqueous electrolyte secondary batteries, such as ignition of leaked electrolyte.
The performance degradation of nonaqueous electrolyte secondary batteries does not occur in the case of cyclic charging and discharging usually carried out. However, in a case where nonaqueous electrolyte secondary batteries are charged by constant-voltage charging at high temperature, there is a problem that significant performance degradation occurs. Further, in a case where nonaqueous electrolyte secondary batteries are charged at high temperature by trickle charging usually used to charge nickel-cadmium secondary batteries and nickel-hydride secondary batteries, more significant performance degradation occurs. For this reason, trickle charging cannot be practically used to charge nonaqueous electrolyte secondary batteries.